Chromating method and component obtained by this method

ABSTRACT

A method of chromating a part including an aluminum-based alloy, the method including the steps of providing the part including a layer based on aluminum oxide on at least one of its surfaces, of immersing the part in a chromating bath including chromium III, and of performing chemical conversion of the part in the chromating bath. The invention also provides a part obtained by the method.

BACKGROUND OF THE INVENTION

The present disclosure relates to providing protection against corrosion for a part comprising an aluminum-based alloy.

Aluminum-based alloys have the advantage of being lightweight. Nevertheless, they can be sensitive to corrosion. Also, it is known to protect parts fabricated from aluminum-based alloys against corrosion, e.g. by performing chemical conversion of the surface of the part.

Such chemical conversion treatment is generally performed by putting the parts into contact with a bath containing hexavalent chromium (or chromium VI or Cr VI). The bath may be made from a solution, e.g. such as the solution commonly referred to by the trademark Alodine® 1200S registered by Henkel. That chemical conversion treatment is a chromating treatment of the aluminum-based alloy, during which the alloy is converted at the surface so as to cause aluminum oxy-hydroxides and aluminum chromates in particular to precipitate at the surface. That treatment makes it possible to produce a coating at the surface of the part serving to increase both the resistance against corrosion and also the resistance against wear of the part made of aluminum-based alloy. Furthermore, that coating serves to conserve electrical conductivity for the coated zone and to provide easy and good quality keying of organic paints.

Naturally, aluminum-based alloys that are exposed to air oxidize, and they form on the surface of the part a passivation layer of aluminum oxide (Al₂O₃), also known as alumina. That layer of aluminum oxide that forms naturally on the surface of the part is referred to as a native oxide layer.

Thus, before performing the chemical conversion treatment, the surface of the part is cleaned in order to remove the passivation layer and expose the aluminum-based alloy. Such cleaning serves to put the aluminum-based alloy into contact with the solution containing hexavalent chromium, and thus to perform the chromating reaction on the part.

Unfortunately, in application of the registration, evaluation, authorization, and restriction of chemicals (REACH) regulation, the use of hexavalent chromium is going to be banned.

Instead of a solution containing hexavalent chromium, one alternative treatment proposes using a solution containing trivalent chromium (or chromium III or Cr III). Nevertheless, it has been observed that the corrosion resistance properties of the coating obtained in that way are generally less good than the corrosion resistance properties of the coating obtained from a solution of hexavalent chromium.

The term “chromating method” is used to mean a method of chemical conversion of a metal part by immersing the part in a chromating bath and by performing chemical conversion of the part in the chromating bath, the chromating bath containing chromium in the form of chromium VI or in the form of chromium III, for example.

OBJECT AND SUMMARY OF THE INVENTION

The present disclosure seeks to remedy these drawbacks, at least in part.

To this end, the present disclosure provides a chromating method for chromating a part comprising an aluminum-based alloy, the method comprising the following steps:

-   -   providing the part including a layer based on aluminum oxide on         at least one surface of the part, which layer is of thickness         that is greater than or equal to 5 nanometers (nm);     -   immersing the part in a chromating bath including chromium III;         and     -   performing chemical conversion of the part in the chromating         bath.

Because of the presence of the layer based on aluminum oxide, while the part is being subjected to chemical conversion in the chromating bath, the coating that is formed presents good corrosion resistance, in particular to salt fog. This layer based on aluminum oxide enables a coating including chromium III and presenting desired anticorrosion properties to be grown in satisfactory manner at the surface of the part.

Thus, instead of seeking to expose the aluminum-based alloy prior to immersion in the chromating bath, as is taught by all chromating methods, this chromating method seeks to promote the presence of an oxide layer on the part before it is immersed in the chromating bath. It can thus be understood that the chromating bath does not include substances for dissolving the layer based on aluminum oxide, as can happen in conventional methods.

The term “aluminum-based alloy” is used to mean an alloy in which the mean content by weight of aluminum is in the majority. It can thus be understood that aluminum is the element in the alloy having the greatest content by weight. By way of example, the aluminum-based alloy has a content by weight of at least 50% aluminum, preferably at least 70% aluminum, still more preferably at least 80% aluminum.

The term “layer based on aluminum oxide” (Al₂O₃), is used to mean a layer in which the mean content by weight of aluminum oxide is in the majority. It can thus be understood that aluminum oxide is the compound in the layer having the greatest content by weight. By way of example, the layer based on aluminum oxide has a content by weight of at least 30% aluminum oxide, preferably at least 40% aluminum oxide, still more preferably at least 50% aluminum oxide.

The chromating bath may also include zirconium.

The presence of zirconium in the chromating bath, e.g. in hexafluorozirconate form, e.g. sodium hexafluorozirconate or potassium hexafluorozirconate, serves to activate the chemical conversion reaction, in particular by forming an Al—F complex with the aluminum at the interface between the part and the coating, the dissolved zirconium then becoming deposited by reduction at the same time as the chromium, with chromium III oxide and zirconium oxide being deposited.

The layer based on aluminum oxide may be a native oxide layer and the part and the layer based on aluminum oxide may be degreased before being immersed in the chromating bath.

The part is generally machined in order to give it a shape close to the finished shape. This machining operation is performed in the presence of a machining fluid, generally an oily fluid. In spite of the presence of oily fluid on the part, a native oxide layer forms at the surface of the part. Specifically, the fluid enables the aluminum-based alloy to oxidize. The oily fluid is removed by degreasing the part.

For example, the part and the layer based on aluminum oxide can be degreased by using a solution that is aqueous and neutral, for example.

It is also possible to envisage using a solvent or mixture of solvents in a vacuum, e.g. perchloroethylene.

It is also possible to envisage using a mixture of azeotropic solvents at atmospheric pressure, such as a mixture of fluorinated ether and chlorocarbon.

The degreasing may be assisted by a method generating ultrasound or cavitation.

The degreasing operation is performed so that the native oxide layer is conserved on the part.

This native oxide layer may have a thickness greater than 5 nm, preferably greater than 10 nm, still more preferably greater than 15 nm, and less than 130 nm, preferably less than 120 nm, still more preferably less than 110 nm.

The layer based on aluminum oxide may be formed by a mechanical method.

It is thus possible to control accurately the thickness of the layer based on aluminum oxide that is present on the part.

Before applying the mechanical method for forming the layer based on aluminum oxide, it is possible to degrease the part so as to clean away any polluting elements on the surface of the part.

The mechanical method may comprise a step of projecting solid particles on the surface of the part.

By projecting solid particles under pressure on the surface of the part, the native oxide layer is removed and the aluminum-based alloy is exposed. The aluminum-based alloy that has just been exposed is highly reactive, and it forms a new layer based on aluminum oxide over all of the surfaces of the part against which solid particles had been projected. This method is also known as “sandblasting”.

The solid particles may be glass beads, e.g. silica beads, or ceramic beads, e.g. zirconia beads.

The mechanical method may comprise a step of abrading the surface of the part with abrasive paper or with a liquid containing abrasive particles.

As with sandblasting, this method exposes the aluminum-based alloy and enables a new layer based on aluminum oxide to be formed on all of the surfaces of the part that have been subjected to abrasion.

It is also possible to envisage adding an oxidizing chemical agent to the abrasive particles in order to promote the formation of the new layer based on aluminum oxide.

The layer based on aluminum oxide is formed by a chemical method comprising a first step of chemical cleaning of the surface of the part followed by a step of chemical or electrochemical oxidation of the surface of the part.

It is thus possible to control accurately the thickness of the layer based on aluminum oxide that is present on the part.

Furthermore, by means of this chemical method, it is possible to treat parts that are complex in shape. Specifically, this chemical method makes use of solutions in liquid form, thus making it possible to reach surfaces that could be difficult to reach by projecting solid particles or by abrasion.

For example, the chemical cleaning step may be performed by spraying the part and the native oxide layer with an acid or alkaline chemical dissolving solution. It is thus possible to dissolve the oxides present at the surface of the part. The part is then dried.

It is also possible to perform at this chemical cleaning step by immersing the part and the native oxide layer in an acid or alkaline chemical dissolving solution.

This chemical cleaning step may be followed by a step of neutralizing the surface of the part before drying.

The chemical cleaning step is followed by a chemical or electrochemical oxidation step in order to promote the formation of the new layer based on aluminum oxide, e.g. by immersing the part in a bath for a time interval lying in the range 1 minute (min) to 30 min.

The layer based on aluminum oxide may be porous.

Thus, the area of contact between the layer based on aluminum oxide and the chromating bath is greater than it would be if the layer based on aluminum oxide were dense. This serves to facilitate the chromating method and to increase its effectiveness.

The layer based on aluminum oxide may be obtained by placing the part in an atmosphere having humidity in the range 30% to 100% and/or of temperature that lies in the range 30° C. to 200° C., e.g. for a time interval lying in the range 5 min to 8 hours (h).

Whether involved in forming the native oxide layer or in forming a new layer based on aluminum oxide, humidity and/or temperature can serve to promote the formation of the layer based on aluminum oxide.

The present disclosure also provides a part comprising an aluminum-based alloy presenting, on at least one surface of the part, a coating including chromium III, the coating having a free surface and having a given thickness, wherein, in a layer of the coating lying in the range 15% to 30% of the given thickness of the coating as measured from the free surface, the amount of aluminum oxide measured in arbitrary units by TOF-SIMS varies at most by plus or minus 50%, preferably by plus or minus 40%, still more preferably by plus or minus 30%.

It can be understood that the amount of alumina in arbitrary units is proportional to its (molar or weight) concentration.

This small variation in the aluminum oxide content close to the free surface of the coating makes it possible to determine that the aluminum oxide content of the coating reaches a high value very quickly, which value is close to the maximum content of aluminum oxide in the coating. This high content of aluminum oxide close to the free surface of the coating is a trace of the layer based on aluminum oxide being present on the part prior to immersion of the part in the chromating bath.

The thickness of the coating may be determined experimentally by determining the content of metallic aluminum of the part starting from the free surface of the part. Once the content of metallic aluminum is constant, it is considered that this is no longer the coating but rather the aluminum-based alloy.

TOF-SIMS stands for time of flight secondary ion mass spectrometry.

The coating may include chromium III in the form of chromium(III) oxide (Cr₂O₃) or in the form of chromium(III) hydroxide (Cr(OH)₃), for example.

The coating that includes chromium III may also include zirconium.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention appear from the following description of implementations of the invention, given as nonlimiting examples, and with reference to the accompanying figures, in which:

FIG. 1 is a partial diagrammatic cross-section view of a part made of aluminum-based alloy and including on at least one surface of the part a layer that is based on aluminum oxide;

FIG. 2 is a diagrammatic cross-section view of a chromating device;

FIG. 3 is a partial diagrammatic cross-section view of the FIG. 1 part after performing the chromating method;

FIGS. 4 and 5 are graphical representations showing how the chemical composition of a coating varies as a function of the thickness of the coating; and

FIGS. 6 to 8 show results of corrosion tests on coatings obtained by chromating.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a part 10 made of aluminum-based alloy. At least one surface of the part 10 includes a layer 12 based on aluminum oxide. The part 10 and the layer 12 based on aluminum oxide form an oxidized part 14.

This layer 12 of aluminum oxide may be a native oxide layer, formed naturally on the surface of the part made of aluminum-based alloy. This layer 12 of aluminum oxide may equally well have been formed by a mechanical method or by a chemical method.

FIG. 2 shows the step of immersing the oxidized part 114 in a vessel 16 containing the chromating bath 18 including chromium III. This chromating bath 18 may also include zirconium.

After performing chemical conversion of the part 10 in the chromating bath 18, a part 10 is obtained that includes, on at least one surface of the part 10, a coating 20 that comprises chromium III, e.g. in the form of chromium(III) oxide (Cr₂O₃) and chromium(III) hydroxide (Cr(OH)₃). When the chromating bath 18 also includes zirconium, the coating 20 also includes zirconium in the form of zirconium oxide (ZrO₂). The part 10 and the coating 20 form a coated part 22 presenting a free surface 24 of the coating 20.

By way of example, the chromating bath 18 may be made from a solution commonly known by the trademarks SurTec® 650 registered by SurTec or Lanthane 613.3 registered by Coventya.

The chromating step itself is well known and is therefore not described in detail.

The layer 12 based on aluminum oxide may have a thickness greater than 5 nm, preferably greater than 10 nm, still more preferably greater than 15 nm, and less than 130 nm, preferably less than 120 nm, still more preferably less than 110 nm. This layer 12 based on aluminum oxide may be the native oxide layer or it may be a new layer based on aluminum oxide obtained by a mechanical method or by a chemical method.

Because of the presence of this layer 12 based on aluminum oxide at the surface of the part 10 made of aluminum-based alloy prior to immersion in the chromating bath 18, after chemical conversion in the chromating bath 18, a coated part 22 is obtained in which the coating 20 has good resistance to corrosion, and in particular to salt fog.

Example 1

A part 10 was taken made of aluminum-based alloy, e.g. grade 7175. The part 10 was machined and thereafter a layer 12 based on aluminum oxide formed on the part 10, the layer having thickness lying in the range 5 nm to 15 nm. The oxidized part 14 was degreased, e.g. using perchloroethylene in the vapor phase in a vacuum. Thereafter, the oxidized and degreased part 14 was immersed in a chromating bath 18 containing chromium III and zirconium. After chemical conversion, a coated part 22 was obtained comprising the part 10 made of aluminum-based alloy and a coating 20.

Example 2

A part 10 was taken made of aluminum-based alloy, e.g. grade 7175. The part 10 was machined and thereafter a layer 12 based on aluminum oxide formed on the part 10, the layer having thickness lying in the range 5 nm to 20 nm. The oxidized part 14 was then sandblasted, i.e. solid particles were projected against the surface of the oxidized part 14 so as to remove the native oxide layer and expose the aluminum-based alloy. Since the aluminum-based alloy that has just been exposed is highly reactive, it formed a new layer 14 based on aluminum oxide over all of the surfaces of the part 10 against which solid particles had been projected. Thereafter, the oxidized and degreased part 14 was immersed in a chromating bath 18 containing chromium III and zirconium. After chemical conversion, a coated part 22 was obtained comprising the part 10 made of aluminum-based alloy and a coating 20.

Example 3

A part 10 was taken made of aluminum-based alloy, e.g. grade 7175. The part 10 was machined and thereafter a layer 12 based on aluminum oxide formed on the part 10, the layer having thickness lying in the range 5 nm to 20 nm. The oxidized part 14 was degreased, e.g. by alkaline degreasing using a solution commonly known by the name Sococlean, for 6 min at 45° C., and it was then chemically cleaned using a solution commonly known under the name Socosurf, for 15 min at 31° C., so that the native oxide layer was removed and the aluminum-based alloy was exposed. The part 10 was then rapidly immersed in the chromating bath 18 containing chromium III and zirconium, so as to limit the formation of a new layer based on aluminum oxide, any layer that might be formed having a thickness strictly less than 5 nm. After chemical conversion, a coated part 22 was obtained comprising the part 10 made of aluminum-based alloy and a coating 20.

FIG. 4 shows the variation in the chemical composition of the coated part 22 of Example 1 as a function of distance from its free surface 24, i.e. as a function of the thickness of the coating.

FIG. 5 is a graph similar to the graph of FIG. 4, for Example 3.

The results of FIGS. 4 and 5 were obtained by time of flight-secondary ion mass spectrometry (TOF-SIMS) analysis. The graphs of FIGS. 4 and 5 have an abscissa axis “T” for erosion time plotted in seconds (s), and an ordinate axis for the molecular composition of the part on a logarithmic scale plotted in arbitrary units as a number of measurement hits “C”. Erosion time is proportional to the distance from the free surface 24 of the coating 20. Thus, a given time corresponds to a distance from the free surface 24 of the coating 20 and to a composition of the coating at that distance.

In FIG. 4, it can be seen that the presence on the part 10 of a layer 12 based on aluminum oxide prior to immersion in the chromating bath 18 gives rise in the coating 20 to a content expressed in a number of detected hits for aluminum oxide (or alumina or Al₂O₃) that is high at a short distance from the free surface 24 of the coating 20. It can be seen that this content, on a logarithmic scale, presents a plateau and that this content varies little within the coating 20.

In FIG. 5, it can be seen that the content of aluminum oxide in the coating 20 does not present a plateau and that it presents a maximum at a greater distance away from the free surface 24

It is also possible to see that the content of aluminum fluoride (AlF₃) varies very differently depending on whether the part 10 had a layer based on aluminum oxide (FIG. 4) or did not have such a layer (FIG. 5).

Since chemical conversion is a method of converting the surface of the part, there is no sharp interface between the body of the part 10 made of aluminum-based alloy and the coating 20. It is thus determined that the transition from the coating 20 to the aluminum-based alloy occurs where the content of metallic aluminum (Al) is relatively constant. Thus, it is determined that the thickness of the coating 20 corresponds to a distance from the free surface 24 of the coating 20 that is equivalent to about 200 s of erosion time and to about 300 s of erosion time, respectively, for FIGS. 4 and 5. In practice, the thickness of the coating 20 is determined by the crossover between the curves for aluminum oxide (Al₂O₃) and for metallic aluminum (Al).

In a coating layer 20 extending from 15% to 30% of the given thickness of the coating 24 of Example 1, as measured from the free surface 24, it can be seen that the amount of aluminum oxide as measured in arbitrary units by TOF-SIMS varies at most by about 15%.

Specifically, since the coating 20 has a thickness corresponding to about 200 s (see FIG. 4), the layer extending from 15% to 30% of the coating, as measured from the free surface 24 of the coating, thus corresponds to the numbers of hits measured in the range 30 s to 60 s. In the range 30 s to 60 s, the number of hits measured for aluminum oxide increases by about 15% (a multiplicative factor of about 1.15).

In the coating layer 20 extending in the range 15% to 30% of the given thickness of the coating 24 of example 3, as measured from the free surface 24, the aluminum oxide content varies by more than 300%. In FIG. 5, the coating 20 presents a thickness corresponding to about 300 s. The layer in the range 15% to 30% of the coating, as measured from the free surface 24 of the coating, thus corresponds to the numbers of hits measured in the range 45 s to 90 s. In the range 45 s to 90 s, the number of hits measured for aluminum oxide increases by more than 300% (a multiplicative factor of about 3.4).

It can be understood that the variation in the aluminum oxide content of the coating layer close to the free surface of the coating, i.e. lying in the range 15% to 30% of the thickness of the coating, as measured from the free surface 24 of the coating 20, makes it possible to determine whether a layer based on aluminum oxide, having a thickness of less than 5 nm, was or was not present on the part 10 made of aluminum-based alloy while the part was being immersed in the chromating bath.

Because coatings are compared as a % of their thickness, it is possible to ignore the erosion parameters used for performing measurement by TOF-SIMS.

FIGS. 6 and 7 show photographs of the results after a salt spray corrosion test for 168 h in compliance with the standard ISO 9227, respectively for Examples 1, 2, and 3.

It can be seen that the coated parts of Examples 1 and 2 do not present corrosion pits, whereas the coated part of Example 3 presents about ten corrosion pits.

Thus, because of the presence of the layer based on aluminum oxide present on the parts made of aluminum-based alloy during the immersion of that part in the chromating bath containing chromium III, a coating 20 is obtained having very good resistance to corrosion.

Although the present disclosure is described with reference to a specific implementation, it is clear that various modifications and changes may be undertaken on those implementations without going beyond the general ambit of the invention as defined by the claims. Also, individual characteristics of the various implementations mentioned above may be combined in additional implementations. Consequently, the description and the drawings should be considered in a sense that is illustrative rather than restrictive. 

1. A chromating method for chromating a part comprising an aluminum-based alloy, the method comprising the following steps: providing the part including a layer based on aluminum oxide on at least one surface of the part, which layer is of thickness that is greater than or equal to 5 nm; immersing the part in a chromating bath including chromium III; and performing chemical conversion of the part in the chromating bath.
 2. The chromating method according to claim 1, wherein the chromating bath comprises zirconium.
 3. The chromating method according to claim 1, wherein the layer based on aluminum oxide is a native oxide layer and wherein the part and the layer based on aluminum alloy are degreased before being immersed in the chromating bath.
 4. The chromating method according to claim 1, wherein the layer based on aluminum oxide is formed by a mechanical method.
 5. The chromating method according to claim 4, wherein the mechanical method comprises a step of projecting solid particles against the surface of the part.
 6. The chromating method according to claim 4, wherein the mechanical method comprises a step of abrading the surface of the part with abrasive paper or with a liquid containing abrasive particles.
 7. The chromating method according to claim 1, wherein the layer based on aluminum oxide is formed by a chemical method comprising a first step of chemical cleaning of the surface of the part followed by a step of chemical or electrochemical oxidation of the surface of the part.
 8. The chromating method according to claim 1, wherein the layer based on aluminum oxide is obtained by placing the part in an atmosphere having humidity in the range 30% to 100% and/or wherein the temperature lies in the range 30° C. to 200° C.
 9. A part comprising an aluminum-based alloy presenting, on at least one surface of the part, a coating comprising chromium III, the coating having a free surface and having a given thickness, wherein, in a layer of the coating lying in the range 15% to 30% of the given thickness of the coating as measured from the free surface, the amount of aluminum oxide measured in arbitrary units by TOF-SIMS varies at most by plus or minus 50%, preferably by plus or minus 40%, still more preferably by plus or minus 30%.
 10. The part according to claim 9, wherein the coating that comprises chromium III also includes zirconium. 